In a mobile communications system mobile or portable user terminals, such as mobile telephones or portable radios, herein collectively referred to as ‘mobile stations’ or ‘MSs’, can communicate via a network infrastructure which generally includes various fixed installations such as base stations (base transceiver stations) or ‘BSs’. Each BS has one or more transceivers which serve MSs in a given region or area, known as a ‘cell’ or ‘site’, by radio communication. The cells of neighbouring BSs are often overlapping. Signals sent from MSs to their serving BS are known as ‘uplink’ signals. Signals sent from a BS to MSs are known as ‘downlink’ signals. Uplink and downlink signals may be sent on different channels, e.g. with different carrier frequencies.
A region or a zone served by an infrastructure including a number of BSs in this way can be considered to be a three dimensional space. At any point in this three dimensional space, a mobile station can be provided with service from one or more of the BSs.
It is desirable to maintain good link reliability between each MS and one of the BSs. It is well known for a MS to monitor signals from various BSs including its current serving BS to determine which BS can provide the best link. Such a procedure is known as ‘cell selection’ or ‘cell re-selection’. Where the MS determines that it should be served by a BS other than its current serving BS it undergoes a procedure known as ‘handover’ or ‘handoff’ to form a service link with the other BS.
It is known for a MS to have a steerable antenna to help to improve the directionality and thereby the path loss of signals sent to and from a chosen BS, with the aim of improving signal strength and reducing interference received at both the BS and the MS.
Steerable antennas are known which employ information describing the known fixed location of the BS and the current location of the MS to calculate the direction of the BS from the MS and thereby to steer the antenna to point in that direction. However, information describing the locations of the BS and the MS alone does not provide an optimum steering direction in a multipath environment. This is because, in such an environment, signals arrive (at the MS or at the BS) at different angles owing to the combined effects of reflection and refraction of the transmitted radiation in addition to direct transmission. The errors so generated will lead to loss of pointing accuracy of the steerable antenna. A searching algorithm may be combined with a calculated pointing direction, but this must be continuously activated and adds a requirement for considerable computing complexity to the MS, which in turn will require increased processing power and therefore will lead to reduced battery life in portable equipment.
Related prior published patent specifications U.S. Pat. Nos. 6,512,481, 6,593,880 and US2004/0104839A, describe a system of obtaining location information by a GPS (Global Positioning System) receiver associated with the MS and computing an antenna pointing direction from the MS to the BS by use of the location information. The system described in these prior specifications does not account for the substantial errors obtained in multipath propagation as described above and would require either acceptance of such errors (and undesirably lower system gain) or addition of an adaptive algorithm with added computational complexity. The system also includes only a single link from the MS to the BS, and requires the MS to use omnidirectional antenna characteristics when changing cells during handover. This requires the computation procedure to start again on acquisition of a link from the MS to another serving BS having a different location. This will cause the MS and BS to have an increased vulnerability to interference and impaired link reliability during handover. The reduction of link reliability at this time may make the system unattractive where high reliability is needed, for example in railway applications where safety critical information may need to be exchanged over the radio link, or in ad hoc systems where the nature of the layout of equipment in the network is subject to change, or in cellular communications systems where multiple cells are required to serve to maintain link reliability over time.